1. Field of the Invention
The present invention relates to a vehicle brake device for applying a friction brake force to a vehicle.
2. Discussion of the Related Art
Heretofore, as one example of vehicle brake devices that apply a friction brake force to a vehicle, there has been known a vehicle brake device shown in FIGS. 11 and 12 of JP2011-240873 A (equivalent of US2011/0285199 A1), for example. In a master cylinder of the vehicle brake device, an input piston and a master piston are held to be separated with a predetermined space therebetween, and when the input piston is moved, a demand brake force is calculated in dependence on the moving amount of the input piston, and a demand friction brake force is calculated by subtracting a demand regenerative brake force from the demand brake force. Then, a pilot pressure corresponding to the demand friction brake force is generated by an electromagnetic valve connected to an accumulator and is inputted to a regulator, which then generates a servo pressure depending on the pilot pressure. Then, the servo pressure is applied to the master piston to move the same, and the a master pressure is generated in a maser chamber to be applied to wheel cylinders, whereby the demand friction brake force is generated.
Electromagnetic valves have a property that is difficult to control the pressure on a downstream side unless a certain degree of flow quantity is supplied as the fluid passing through the electromagnetic valve. However, brake fluid is an incompressible fluid, and thus, when the brake fluid passing through the electromagnetic valve is extremely little in flow quantity, the pilot pressure to the regulator overreacts to the operation of the electromagnetic valve. For this reason, there is an anxiety that when the electromagnetic valve is opened to raise the pilot pressure, the pilot pressure raises abruptly and overshoots a desired pilot pressure. Like this, in the vehicle brake device in the prior art, it is difficult to control the pilot pressure accurately, and hence, a problem arises in that the servo pressure and the master pressure are difficult to control accurately.